Terms like "sense" and "tell" are a bit misleading. It's very much a chemical/mechanical interaction that's automatic. Rather like soap bubbles "sensing" when they've reached the surface of the water.

Plants contain a protein called phototropin, which is activated by light. When it's activated, it changes the shape and alignment of the "skeleton" of the cell, making it more cube-shaped as opposed to long and skinny.

That means the light side of a plant gets shorter, while the dark side remains long. The dark side also grows slightly faster, on a count of having more cells there (you can fit more skinny cells side-by-side than wide cells), and so the plant angles and grows toward the light.

I would speculate that [reflected] light also has a unique color (wavelength) distribution that a plant could sense and respond to

It seems as far as we can tell, trees can detect "far red" spectrum light, suspected to be done via phytochromes, and that spectrum of light is in higher quantities when closer to other tree leaves because it gets reflected off.

They detect that, and don't grow as much in that direction since it would cause diminishing returns.

The reflected light of other leaves wouldn’t cause photosynthesis since it only has wavelengths that the chloroplasts reflect. They wouldn’t have any light to absorb, or at least a lot less.

I imagine it’s like expecting regular soda and getting diet.